This application claims the priority of German patent document 198 53 595.3, filed Nov. 20, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for determining the oxygen content of a gas.
Increasingly stringent auto emission standards and pressure to reduce fuel consumption of internal combustion engines are compelling automobile manufacturers to develop new concepts for internal combustion engines. It has been shown that the above two requirements can best be harmonized by operating the internal combustion engine with excess air (that is, in a so-called xe2x80x9cleanxe2x80x9d mode, with an air/fuel ratio xcex greater than 1). This modern xe2x80x9clean conceptxe2x80x9d requires that the oxygen level of the exhaust be known precisely. However, the principle of the conventional potentiometric oxygen sensor (lambda probe) can be implemented only at high cost for the high oxygen concentrations that occur in such lean exhausts.
For measuring exhaust oxygen content in the lean range, amperometric probes on the limiting current principle (xe2x80x9climiting current probesxe2x80x9d) have been proposed. Such probes can be made of a material that conducts oxygen ions, as disclosed for example in Kleitz M., Siebert E., Fabry P., Fouletier J.: Solid-State Electrochemical Sensors; In: Sensors. A Comprehensive Survey; Chemical and Biochemical Sensors Part I. Gxc3x6pel W. et al. (pub.), VCH-Verlag, Weinheim, 1991, pages 341-428; see also German patent documents DE 44 47 033, DE 44 08 361, DE 43 91 637, and DE 27 11 880. It is also possible however to utilize the oxygen partial pressure dependence of the electrical conductivity of a metal oxide material as a sensor effect to produce a sensor whose electrical resistance R, due to the oxygen partial pressure pO2 of the exhaust, gives information on the oxygen content of the exhaust. See Howarth D. S., Micheli A. L.: A Simple Titania Thick Film Exhaust Gas Oxygen Sensor, SAE 840140, 1984; Schonauer U.: Strontium Titanate Oxygen Sensors in Thick Film Technology; Dissertation, Karlsruhe 1990; Gerblinger J.: Oxygen Sensors Based on Sputtered Strontium Titanate Layers; Dissertation, Karlsruhe 1991; and Schonauer U.: Thick Film Oxygen Sensors Based on Ceramic Semiconductors. Technisches Messen 56 [6] 260-263, 1989. Doped titanium oxide (TiO2) and strontium titanate (SrTiO3) have been investigated in particular depth, as such titanium oxides have sufficient chemical stability to withstand the harsh operating conditions in the exhaust line of an internal combustion engine. However, the electrical resistance of sensors made from these compounds, like most other metal oxides, has a very high temperature dependence, requiring expensive heat regulation combined with comprehensive design measures to compensate for the effects of sudden temperature changes.
It has also been proposed to use cuprates (e.g. La2CuO4+d) because their electrical conductivity is independent of temperature precisely in the range of high oxygen level, i.e, at xcex greater than 1. See German patent documents DE 42 02 146, DE 42 44 723, DE 43 25 183 and Blase R.: Temperature-dependent Oxygen Sensors with Short Adjustment Time Based on La2CuO4+d Thick Layers; Dissertation, Karlsruhe 1996. However, cuprates are unsuitable for use in the exhaust line as they are chemically somewhat unstable and decompose at high temperatures and/or low oxygen partial pressures (e.g. short-term operation with xe2x80x9crichxe2x80x9d mixtures, (xcex less than 1).
Lanthanum ferrites doped with alkaline earths have far greater chemical stability than cuprates. (See German patent document DE 44 18 054.) By comparison to SrTiO3, the temperature dependence of their electrical conductivity is lower in the lean exhaust range as well (xcex greater than 1). However, sensors made from these materials have greater temperature dependence on electrical resistance than sensors based on cuprates.
European patent document EP 0 062 994 proposes partial replacement of the titanium (Ti) in SrTiO3 by iron (Fe). Sensors made from the compound SrTi0.7Fe0.3O3xe2x88x92d in lean atmospheres above 500xc2x0 C.-600xc2x0 C. have far lower temperature dependence of electrical resistance, but their oxygen partial pressure dependence is according to Rxcx9cpO2xe2x88x92⅕.
Advantageously, such resistive oxygen sensors are made by thick film technology. A heating resistor film is applied to one flat side of an electrically nonconducting substrate, and a sensitive functional layer and possibly a temperature-measuring resistor are applied to the other flat side. This sensor arrangement is disposed in a protective housing and provided with a lead.
The resistive principle has the advantage that the oxygen content of a gas can be determined by means of a simple resistance measurement. It has the disadvantage however that in resistance measurement not only the material properties in the form of the specific electrical resistance but also the geometry of the sensitive functional layer are involved. The width and length of a film can be created well and reproducibly. However, the manufacturing of a film thickness that is exactly reproducible involves expensive process technology. Another difficulty arises when such sensors are to be operated in harsh environments such as in the exhaust line of an automobile or a power plant. Because of abrasion and other mechanical effects such as chipping on a microscopic scale, the geometry of the film changes and thus the sensor characteristic alters over time.
Another major disadvantage is based on the morphology of the sensitive films. Such polycrystalline functional layers have grain boundary layers that have different electrical behaviors to the insides of the grains. This phenomenon is even used as a measuring effect, for example in gas sensors made from SnO2 films. See Ruhland B., Becker T., Mxc3xcller G.: Gas-kinetic Interactions of Nitrous Oxides with SnO2 Surfaces. Sensors and Actuators B 50 85-94, 1998 and Mosley P. T.: Solid-State Gas Sensors. Meas. Sci. Technol. 8 223-237, 1997. In operation, a changing grain boundary layer leads to undesirable drift of the sensor characteristic and sometimes to losses of sensitivity as well.
For precise results, particularly in the lean range, four-wire technology must be used for measurement at the sensitive functional layer, as this is the only way for contact resistances between the electrode and the functional layer to be calculated. This is important because contact resistances usually change with the oxygen partial pressure of the gas atmosphere as well. For this reason, additional expensive electrical connections are necessary.
Measuring an electrical voltage, for example with a simple limiting current sensor, is even simpler than using an electrical resistance. In this case, however, the diffusion constant of the diffusion barrier, which is yet another temperature- and material-dependent parameter, is a factor in the measured signal. Moreover, such a sensor is not easy to build and is very expensive because of the expensive multilayer technology.
One object of the invention is to provide a measuring method and apparatus for determining the oxygen content of gases, that overcomes the disadvantages of the prior art referred to above.
This and other objects and advantages are achieved by the detection method and apparatus according to the invention, in which a functional material in the form of a layer or film of a semiconducting metal oxide, whose thermoelectric power can be represented as a function of partial oxygen pressure, is subjected to the gas to be analyzed. A temperature difference is generated between two points on the functional material, and the voltage difference between the two points is measured. Since such voltage difference depends on the thermoelectric power of the functional material, it reflects the oxygen concentration of the gas to be analyzed. The voltage difference is also referred to hereinafter as the output signal of the transducer.
In a preferred embodiment, the temperature difference between the two points on the functional layer that are at different temperatures is determined by suitable direct or indirect measuring methods.
Another embodiment of the invention also considers the temperature of the functional layer.
If the transducer is operated above a temperature at which the thermoelectric power of the functional layer is a function of the oxygen partial pressure of the environment, the oxygen content of the gas surrounding the sensor can be determined from the sensor output signal, possibly considering the temperature difference between the two points at different temperatures on the functional layer.
The method and apparatus according to the invention can be used in particular to:
determine the oxygen content of exhaust gases f from furnaces (power plants and domestic);
determine the oxygen content of exhaust gases from internal combustion engines such as gasoline or diesel engines;
in medicine, detect the oxygen content of air exhaled from the body.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying.